feat(cache): remove cache operator

Because of all of the confusion around the `cache` operator, this removes the operator
from RxJS head of the 5.0.0 release. This is done for a few reasons\:

1. The name `cache` implies it supports all sorts of caching strategies, when this is pretty limited
2. Behavior around unending and hot observables is hard to reason about
3. Once we have a properly designed `cache` operator we can add it at a later time and it will not be a breaking change

Author: benlesh

spec/operators/cache-spec.ts

@@ -44,7 +44,6 @@ import './add/operator/bufferCount';
 import './add/operator/bufferTime';
 import './add/operator/bufferToggle';
 import './add/operator/bufferWhen';
-import './add/operator/cache';
 import './add/operator/catch';
 import './add/operator/combineAll';
 import './add/operator/combineLatest';